The present invention relates to production of thin film magnetic heads. In particular, the invention relates to aligning the upper and lower pole tips in a thin film magnetic head using a sacrificial layer yielding a contoured pole face which provides reduced undershoot in a readback signal.
Thin film magnetic read/write heads are used for magnetically reading and writing information upon a magnetic storage medium such as a magnetic disc or a magnetic tape. It is highly desirable to provide a high density of information storage on the magnetic storage medium.
Recording systems typically become more cost effective by providing areal densities which are as high as possible for a given recording surface. In the case of rotating disc drives (both floppy and hard disc), the areal density is found by multiplying the number of bits per unit length along the track (linear density in units of bits per inch) by the number of tracks available per unit length in the radial direction (track density in units of tracks per inch).
The demand for increased storage density in magnetic storage media has led to reduced magnetic head dimensions. Magnetic heads are now fabricated in a manner similar to that used for semiconductor integrated circuits in the electronic industry.
During fabrication, many thin film magnetic heads are deposited across the surface of a wafer (or substrate). After the layers are deposited, the wafer is "diced" or sliced into many individual thin film heads, each carried by a portion of the wafer so that an upper pole tip, a lower pole tip, and a gap are exposed. The pole tips and the gap (and the portion of the substrate which underlies them) are then lapped in a direction generally inward, toward the center of the thin film head, to achieve the desired dimensions. This lapping process is a grinding process in which the exposed portion of the top and bottom pole tips and the gap are applied to an abrasive, such as a diamond slurry. Electrical contacts are connected to conductive coils. The completed head is attached to a carrying fixture for use in reading and writing data on a magnetic storage medium such as a computer disc.
In operation, the exposed upper and lower pole tips are positioned near a moving magnetic storage medium. During the read operation, the changing magnetic flux of the moving storage medium impresses a changing magnetic flux upon upper and lower pole tips. The magnetic flux is carried through the pole tips and yoke core around the conductor coil. The changing magnetic flux induces an electrical voltage across the conductor coil which may be detected with electrical detection circuitry. The electrical voltage is representative of the changing magnetic flux produced by the moving magnetic storage medium.
During a write operation, an electrical current is caused to flow in the conductor coil. This electric current induces a magnetic field in top and bottom magnetic poles and causes a magnetic field across the gap between the upper and lower pole tips. A fringe field extends in the vicinity beyond the boundary of the pole tips and into the nearby magnetic storage medium. This fringe field may be used to impress magnetic fields upon the storage medium and magnetically write information.
The highest track density achievable is strongly influenced by the accuracy and precision of alignment of upper and lower pole tips and their width. Magnetic pole tips typically have a pole thickness in the range of about one micrometer to about five micrometers depending upon design criteria, i.e. a thicker pole for better overwriting efficiency and a thinner pole for increased resolution capability during the readback operation.
As track density increases, currently approaching and exceeding 2400 tracks per inch, the alignment between the upper and lower pole tips in thin film magnetic read/write heads has become critical. At such a high storage density, design criteria require magnetic transducers in which the bottom pole tip width is very nearly the same as the top pole tip width. Top and bottom pole tips should also be in close alignment. At these small dimensions, alignment between the pole tips of a head becomes critical, particularly as dimensions of the pole tips approach the tolerance and definition limits of the deposition techniques. A technique which provides better pole alignment begins with a top pole, bottom pole and a gap area separating the top and bottom poles, all fabricated substantially wider than desired. A narrower mask layer is then deposited upon the upper pole. The structure is then aligned using a material removal process ("milling") such as ion milling or reactive ion milling in which high energy ions bombard the pole tip region to remove the excess material (top pole, bottom pole and gap material) that extends beyond the edges of the mask layer. The mask layer protects only a portion of the top pole, bottom pole and gap so that the width of the completed pole tips is approximately the same as the width of the mask layer.
The noted alignment technique suffers from a number of drawbacks. The mask layer is difficult to remove from the pole tip structure after the milling process. To ensure adequate protection of the pole tips during milling, the mask must be very thick to withstand the milling process. A thick mask, however, decreases the ability to control the shape of the pole tips. Furthermore, if the remaining mask material is stripped away following milling, the delicate structure of the thin film head may be damaged. If, on the other hand, the mask layer is made thinner to improve process control and facilitate removal of the mask following ion milling, the risk of damaging the pole tip structure during milling is increased.
During readback of magnetically stored information, the thin film head provides an electrical output signal which is representative of both the relative strength of the magnetization in the media, and the magnetic field pattern of the read head. It is highly desirable to provide the highest level of information storage density possible for a given magnetic storage system. Unfortunately, increased storage density leads to a lower signal-to-noise ratio for the sensed signal from a given disk. The readback signal comprises a series of superimposed symbols whose existence and location are used to represent digital information.
Signal recovery errors will result if the detection circuitry is confused in one of the following ways:
1. Detecting a symbol that was not written; PA1 2. Rejecting a symbol that was written; PA1 3. Placing a written symbol in a wrong clock cell.
Cost effective detectors presently in use are confused by leading and trailing undershoots in the isolated readback pulses just as easily as they are confused by noise. This is because for an arbitrary recorded pattern, the undershoot readback waveform from one recorded transition can interfere with the main pulse of another transition and result in one of the above signal recovery errors.
Therefore, the undershoots reduce the maximum recording density which may be achieved in a magnetic storage system. Undershoot occurs due to discontinuities in the magnetic readback flux path characteristic of the finite pole lengths of the thin film recording head. Rather than trying to compensate for the undershoots in the data signal using sophisticated electronic decoding methods, it would be highly desirable and a significant contribution to the art to provide a thin film magnetic head which minimizes the leading and trailing undershoots in the isolated readback pulse.
An accurate and precise method of aligning pole tips and reducing undershoot in the readback signal would be an important contribution to the art.